1. Field of the Invention
This invention relates to permanent magnets and more particularly, to a method for making permanent magnets of polycrystalline manganese-aluminium-carbon alloys suitable for multipolar magnetization. Also, it relates to permanent magnets obtained by the method.
2. Description of the Prior Art
Mn--Al--C alloy magnets are mainly constituted of the structure of ferromagnetic face-centered tetragonal phase (.tau. phase L1o type superstructure) and contain carbon as their essential component element. The magnets include those magnets of ternary alloys free of any additive elements except for inevitable impurities and quaternary or multicomponent alloys which contain small amounts of additive elements. By the term "Mn--Al--C alloy magnet" used herein are meant magnets of all the alloys including quaternary or multi-component alloys as well as ternary alloys.
Known methods of making Mn--Al--C alloy magnets include, aside from those methods using casting and heat treatments, a method which comprises a warm plastic working process such as warm extrusion. The latter method is known as a method of making an anisotropic magnet which has excellent properties such as high magnetic characteristics, mechanical strength and machinability.
On the other hand, Mn--Al--C alloy magnets for multipolar magnetization can be made by several techniques including a technique using isotropic magnets or compressive working, and a technique in which a uniaxially anisotropic polycrystalline Mn--Al--C alloy magnet obtained by a known method such as warm extrusion is subjected to warm free compressive working in a direction of easy magnetization, i.e. a compound working method.
However, the compound working method includes a free compressive process. When a work is assumed to be in the form of a solid cylinder, a too high ratio L.sub.o /D.sub.o where D.sub.o represents a diameter of the cylinder and L.sub.o represents a length of the cylinder will produce the problem of failure of the work due to buckling.
This will impose the limitation that a ratio L/D in which D and L, respectively, represent a diameter and a length of a work after the free compression cannot be made large. Accordingly, it is necessary to use several magnets in order to obtain a long magnet with a high L/D ratio. For instance, Mn--Al--C alloy magnets are so excellent in mechanical strength and machinability that they can be applied in the form of a rod of monolithic magnet for the purpose of outer lateral magnetization. However, long magnets cannot be obtained using any hitherto known methods. Thus it is the usual practice that a plurality of Mn--Al--C alloy magnet pieces are machined into hollow cylinders and are joined together for practical applications. Long magnets have another advantage in that magnets of any shorter sizes can be obtained by cutting the long magnet into pieces of a desired length.